PHP Character Encoding Requirements

which has ASCII-compatible (ISO646 compatible) mappings for the
characters in range of 00h to
7fh.

A multibyte encoding,

which has ASCII-compatible mappings for the characters in range of
00h to 7fh.

which don't use ISO2022 escape sequences.

which don't use a value from 00h to
7fh in any of the compounded bytes
that represents a single character.

These are examples of character encodings that are unlikely to work
with PHP.

JIS, SJIS, ISO-2022-JP, BIG-5

Although PHP scripts written in any of those encodings might not work,
especially in the case where encoded strings appear as identifiers
or literals in the script, you can almost avoid using these encodings
by setting up the mbstring's transparent encoding
filter function for incoming HTTP queries.

Note:

It's highly discouraged to use SJIS, BIG5, CP936, CP949 and GB18030 for
the internal encoding unless you are familiar with the parser, the
scanner and the character encoding.

Note:

If you are connecting to a database with PHP, it is recommended that
you use the same character encoding for both the database and the
internal encoding for ease of use and better
performance.

If you are using PostgreSQL, the character encoding used in the
database and the one used in PHP may differ as it supports
automatic character set conversion between the backend and the frontend.